The present invention relates to the non-destructive control of moving products, especially a steel strip in the course of its manufacturing process in the iron-and-steel industry.
More specifically, the invention relates to the ultrasonic detection of defects in such a strip by subjecting it to guided vibratory waves, for example so-called Lamb waves.
Lamb waves are ultrasonic vibrations which propagate in the thickness of the product to be inspected, being guided by opposed plane faces of the product. In the case of flat products, such as a metal strip, the waves, channeled by the two parallel plane faces of the product to be inspected, propagate transversely to the direction of movement, and therefore along the width of the strip, from a point of emission, chosen for placing the transducer, to an edge where they are reflected, thereby producing a strong echo signal. Any defect, for example a nonmetallic inclusion, present in the strip or emerging on the surface thereof during passage of the wave represents a local discontinuity of the material transversed which responds with a signal, the intensity of which depends on the size and nature of the defect and the moment of appearance of which, between the emission echo and the edge echo, is representative of the point on the strip where the defect occurs.
The industrial practice is generally to use two transducers, arranged one after the other in the direction of movement of the strip. Each transducer is placed near a different edge of the strip and emits in the direction of the opposite edge. In this way, the strip is scanned twice in a short time interval and in opposite directions, thereby improving the reliability of the results of the analysis, as well as coverage of the area probed.